I read the article, but I don't see why this has to be built on top of a blockchain. What's wrong with plain old relational databases? The currency is issued by

I read the article, but I don't see why this has to be built on top of a blockchain. What's wrong with plain old relational databases? The currency is issued by the central bank, if the central bank says that this transaction is voided, who's gonna stop them? Blockchains are a solution to byzantine consensus, but with a central authority you can always trust the central bank (in the consensus problem sense of trust), so I don't see what problem gets solved by moving to a slower and worse database.